A family member frequently drives around in the van all by themselves, which I'm sure is an enormous waste of gasoline. Removing one or both of the rear seats will help reduce the weight of the vehicle and improve the efficiency a tiny bit.
That said, I was wondering if doing that would cause handling issues due to the centre of gravity being farther forward. Also, winter is long and icy here, so rear traction is an issue 5 months of the year.